This invention relates to a process for recovering a polymer from a dispersion in a liquid medium using a twin-screw extruder of special design from which solid polymer is extruded substantially free from the liquid. This process is useful for dispersions of both rubbery and thermoplastic polymers in either aqueous or nonaqueous liquids.
It is known to recover polymers from their slurries or latices by extrusion. Many types of commercial extruders have been employed for this purpose, including, for example, simple screw extruders, twin screw extruders, or the so-called French press. Because some of this equipment operates under a rather high pressure, it is also necessary to feed the liquid dispersions of polymers under high pressure. However, under those conditions, especially when polymer is subjected to high shear, for example, in screw pumps, rubber latices tend to coagulate and clog the extruder inlet. Thermoplastic polymers melt to viscoelastic masses that are difficult to convey. In some other equipment, sticky thermoplastic or elastomeric material cannot be successfully pressed or conveyed.
Chloroprene polymers and copolymers are usually isolated from their aqueous latices by the freeze-drum technique, which unfortunately has a high demand of energy. Extrusion has a greater potential and general applicability because of its lower energy requirement and lesser environmental impact. When properly designed and operated, it can also provide an overall cost advantage. Some prior art extrusion processes suffered, however, from shortcomings. For example, frequently chloroprene polymer latices were not completely coagulated when non-acidic coagulants were used; while, when acidic coagulants were used, the final polymers had insufficient alkaline reserve to provide adequate storage stability.
It thus is desirable to provide an improved extrusion process for isolating polymers from their dispersions in liquids, wherefrom the above disadvantages of prior art processes would be eliminated.